Inclusion Criteria

Patients may be eligible for this study if they:

• Are HIV-positive.
• Are at least 18 years old.
• Have a viral load (level of HIV in the blood) of at least 500 copies/ml but no more than 100,000 copies/ml within 45 days of study entry.
• Have been taking the following anti-HIV drug combination for at least 12 weeks before study entry: 2 NRTIs plus amprenavir (APV), nelfinavir (NFV), saquinavir (SQV), or NFV plus SQV.
• Are naive to at least 1 NRTI. This means that there is at least 1 NRTI that the patient has not taken for more than 14 days. In the case of lamivudine (3TC), naive means that the patient has never taken this drug.
• Are willing and able to drink 1.5 liters (a little over 1.5 quarts) of water or other fluids a day.
• Agree to use an effective barrier method of birth control (such as condoms) during the study and for 3 months after.

Exclusion Criteria

Patients will not be eligible for this study if they:

• Have taken protease inhibitors other than APV, NFV, SQV, or NFV plus SQV.
• Are resistant to the effects of IDV or RTV, as shown by a blood test. (Patients whose viral load is between 500 and 1,000 copies/ml will not need to be tested.)
• Have any active opportunistic (AIDS-related) infection in the 14 days before study entry.
• Have any medical condition or history of disease that would prevent them from completing the study or put them at risk.
• Have cancer that requires chemotherapy.
• Have an active infection that requires treatment in the 14 days before study entry.
• Have a fever for a week or more in the 30 days before study entry.
• Have taken nonnucleoside reverse transcriptase inhibitors (NNRTIs) in the 30 days before study entry.
• Have received a vaccine in the 21 days before study entry.
• Have received an experimental drug or a drug that affects the immune system in the 30 days before study entry.
• Have taken or plan to take certain other medications that may affect the study.
• Are pregnant or breast-feeding.